1. Field of the Invention
The present invention relates to an electrically erasable and programmable nonvolatile semiconductor memory device.
2. Description of the Related Art
A basic unit of an electrically erasable and programmable nonvolatile semiconductor memory is a memory cell whose structure is as follows: an N-type source region and a N-type drain region disposed on both side of a channel region and both disposed on a P-type silicon substrate; a tunnel region disposed on a part of the drain region; a floating gate electrode formed on a tunnel insulating film formed of a thin silicon oxide film or a composite film of a silicon oxide film and a silicon nitride film having a thickness equal to or less than a hundred angstrom; and a control gate electrode formed on a control insulating film formed of a thin insulating film and having a strong capacitive coupling with the floating gate electrode. The floating gate electrode is isolated electrically from the circumference and is capable of storing electric charges inside of it.
The floating gate electrode and the control gate electrode are both extended upon the channel region, and the conductance of the channel region varies according to the potential of the floating gate electrode. Accordingly change of the electric charges in the floating gate electrode can prompt a nonvolatile memory of information. Application of a voltage of about 15 volts or more to the drain region and also to the tunnel region against the control gate electrode generates a tunnel current which ejects electrons stored in the floating gate electrode to the drain region through the tunnel insulating film formed in the tunnel region, and it is also possible to inject electrons into the floating gate electrode vice versa.
As described above change of the electric charge in the floating gate causes the functions as a nonvolatile memory. A lot of memory cells having such functions can be configured in columns and rows to form a memory array to have a large volume nonvolatile semiconductor memory device.
At this point the tunnel region having the tunnel insulating film through which electrons transport is especially important. On one hand the tunnel insulating film enables a quite lot of rewriting of memory information up to some hundred thousands times, and on the other hand it plays a dominant role against the demand for long term storage of memory information (retention of the electric charges) for a few decades.
As an improvement in reliability of the tunnel region and the tunnel insulating film, a device in which a tunnel region having a different impurity concentration is formed next to the drain region to improve the program/erase characteristic (endurance characteristic) or the retention characteristic is proposed as in, for example, Japanese Patent Publication H01-160058A.